Abstract
Thermomigration in flip chip solder joints accompanied by electromigration had been reported in several years; however, not many studies focus on the influence of ambient temperature on thermomigration of solder joints. To understand how ambient temperature affects the microstructural evolution of solders, a constant current of 1A was applied on the composite 95%Pb5%Sn- eutectic SnPb flip chip specimens to establish a temperature gradient for five ambient temperatures, i.e. 100˚C, 125˚C, 150˚C, 160˚C and 175˚C. The results show that no microstructural evolution of unpowered solders was found for samples tested at ambient temperature of 100˚C and 125˚C; whereas thermomigration induced phase separation was observed for samples tested at 150, 160 and 170 ˚C, suggesting that the thermomigration only occurred when the ambient temperature was higher than 150˚C. We explained this phenomenon by the effect of back stress. After considering the contribution of a stress gradient and a temperature gradient on the mass transport, the threshold temperature difference needed to trigger thermomigration is found to be decreased with increasing the ambient temperature. In addition, given that temperature gradient of 2580 ˚C/cm the theoretical calculation indicated that a threshold temperature to trigger thermomigration is about 150˚C, which is consistent with our experimental results. For the powered solder bumps, since no thermomigration occurred at 100˚C and 125˚C, only electromigration happened and the asymmetrical phase distribution of the pair of bumps was observed due to polarity effect. However, when tested ambient temperature is 150 ˚C and 160 ˚C, the symmetrical phase distribution of the pair of bumps was found. We proposed that this symmetrical phase separation for bumps test 150 ˚C and 160 ˚C is because the driving force of thermomigration is larger than that of electromigration. When the ambient temperature further increases to 175 ˚C, the electromigration become dominant, thus asymmetrical phase distribution of the pair of bumps was observed again. Furthermore, electromigration induced void formation was found at the solder/ IMC interface near the substrate side instead of chip side, which is believed to be because the samples used in this study possess thicker Ni UBM on the chip side and lack Ni diffusion barrier on the substrate side.